1. Field of the Invention
This invention is related to the field of communication systems, and in particular, to systems and methods of providing discrete amplification of an optical signal.
2. Statement of the Problem
Many communication companies use fiber optic cabling as a media for transmitting data because of its high-bandwidth capacity. Fiber optic cables reliably transport optical signals over long distances. Over a distance, an optical signal attenuates in the fiber due to Rayleigh scattering. The attenuation may be recovered by an optical amplifier, however, the optical amplifier adds noise to the optical signal. The noise accumulation on the optical signal can especially be a problem for ultra long haul transmissions that have a high bit rate.
Optical amplifiers may be discrete amplifiers or distributed amplifiers. Distributed amplifiers use the transmission fiber, that is carrying the optical signal, as a gain medium. Discrete amplifiers do not use transmission fiber as a gain medium, but use another type of fiber or component as the gain medium.
One type of discrete amplifier is an Erbium-Doped Fiber Amplifier (EDFA). In an EDFA, an Erbium-doped fiber receives optical signals from a transmission fiber. A pump laser transmits a 980 nm laser beam onto Erbium-doped fiber concurrently as the optical signals travel over the Erbium-doped fiber. The properties of the Erbium-doped fiber act to absorb the laser beam and generate a gain in the optical signals using the absorbed laser beam. In this example, the Erbium-doped fiber acts as the gain medium, not the transmission fiber. Unfortunately, EDFA's have a limit on the gain bandwidth they can produce and cannot effectively be used for ultra wide band transmissions.
Another type of discrete optical amplifier is a Raman amplifier. In a discrete Raman amplifier, a fiber span within the Raman amplifier receives optical signals from a transmission fiber. The fiber span may be a highly doped fiber, such as a dispersion compensating fiber. A Raman pump laser backward pumps a laser beam onto the fiber span carrying the optical signals. Based on the “Raman Effect”, the laser beam generates a gain in the optical signals traveling on the fiber span. For instance, a 1480 nm laser beam, transmitted over a fiber span carrying optical signals, generates a gain in the optical signals in the range of 1565-1600 nm. The discrete Raman amplifier provides a wider gain bandwidth and allows for replacement of high-powered EDFAs. However, the discrete Raman amplifier generates a higher noise figure than EDFAs.
Raman amplifiers can also be used for distributed amplification. Designers have improved the noise figure problems for distributed Raman amplification using a second order pump. One particular pumping scheme was described in a paper entitled “Transparent 80 km Bi-Directional Pumped Distributed Raman Amplifier with Second Order Pumping”, which was authored by Karsten Rottwitt et. al. and published in ECOC '99, Sep. 26-30, 1999 (Rottwitt paper), which is incorporated herein by reference into this application. The Rottwitt paper describes a distributed optical amplifier using a second order pump in addition to a first order pump. The first order pump backward pumps a laser beam onto a transmission fiber and the second order pump forward pumps a laser beam onto the transmission fiber. The transmission fiber acts as the gain medium, thus describing distributed amplification. To improve the noise figure, the first order pump is set to a power of 200 mW at 1455 nm, while the second order pump is set to a power of 800 mW at 1366 nm. By adding the second order pump, the gain is generated earlier in the length of the transmission fiber resulting in an improved noise figure. This configuration is illustrated below in FIG. 1.
One problem with the distributed optical amplifier described in the Rottwitt paper is that it only describes distributed Raman amplification, and does not effectively describe a discrete optical amplifier. Discrete optical amplifiers may be preferred over distributed optical amplifiers for some applications or configurations. Another problem with the distributed optical amplifier described the Rottwitt paper is that only the noise figure is considered when setting the pump powers of the first order pump and the second order pump. The pump powers may affect other performance factors. For instance, as the power of the second order pump increases, the gain in the transmission fiber occurs earlier in the transmission fiber. The earlier the gain occurs, the higher the average signal power over the length of the transmission fiber. The higher the average signal power over the length of the transmission fiber, the higher the fiber non-linearities that may negatively affect the optical signal. Thus, the Rottwitt paper fails to consider other performance factors when setting the pump powers.